get_appengine.py: fix for go_appengine
get_appengine.py: fix for go_appengine

we should preserve timestamp of files in go_appengine SDK.
otherwise, goapp test won't work.

BUG=405946

Review URL: https://codereview.chromium.org/489703003

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra